1. Field of the Invention
The present invention relates to a photographing lens system provided in a photographic camera, an electronic still camera or a video camera, and particularly relates to a zoom lens system of high zoom-ratio, which is miniaturized, and achieves superior optical quality over the entire zooming range.
2. Description of Related Art
In recent years, in the product-field of photographic cameras, electronic still cameras and video cameras, a zoom lens has been typically used; and, the zoom lens has been more strongly demanded to have higher optical quality, a higher zoom ratio, and to be more compact in size.
A well-known zoom lens system suitable for achieving high optical quality and a high zoom-ratio includes a first lens group having a positive refractive power (hereinafter, a positive first lens group), a second lens group having a negative refractive power (hereinafter, a negative second lens group), a third lens group having a positive refractive power (hereinafter, a positive third lens group), and a fourth lens group having a positive refractive power (hereinafter, a positive fourth lens group), in this order from the object. In the zoom lens system with the above lens arrangement, upon zooming from the short focal length extremity (wide-angle extremity) to the long focal length extremity (telephoto extremity), the distances between each of these lens groups vary.
Furthermore, typically in zoom lens systems having a large number of lens groups, if the refractive power of each lens group is made stronger, the traveling distance of each lens group along the optical axis upon zooming can be reduced, and hence, a compact zoom lens system can be achieved.
On the other hand, strengthening the refractive power of each lens group, however, increases aberration fluctuations of each lens group when zooming is performed, and tends to increase residual aberrations of each lens group. Accordingly, if the refractive power of each lens group is set inappropriately, such as being made stronger without being through suitable design-reviewing process, etc., it is very difficult to achieve a compact zoom lens system having a high zoom-ratio and a high optical quality.
Therefore in order to simultaneously achieve conflicting requirements, such as a higher zoom-ratio and higher optical quality, in a zoom lens system, it becomes important to determine the refractive power of each lens group appropriately so that the refractive power-balance can be well maintained over the lens groups.
For example, in the zoom lens system proposed in Japanese Unexamined Patent Publication No. 2006-171615, a lens arrangement of a positive lens group, a negative lens group, a positive lens group and a positive lens group in this order from the object, is employed. However, since the refractive power balance among the lens groups is inappropriately set, the Petzval sum is increased in the negative direction. Consequently, astigmatic difference and field curvature increase, and optical quality with respect to the off-axis region becomes insufficient.
The zoom lens system proposed in Japanese Unexamined Patent Publication No. 2008-225328 also employs a lens arrangement having a positive lens group, a negative lens group, a positive lens group and a positive lens group, in this order from the object. However, since the lens arrangement of the fourth lens group is inappropriate, lateral chromatic aberration is undercorrected, especially at the long focal length extremity, and color flare undesirably and largely occurs in the off-axis region.
The zoom lens system proposed in Japanese Unexamined Patent Publication No. 2008-268787, likewise, employs a lens arrangement having a positive lens group, a negative lens group, a positive lens group and a positive lens group, in this order from the object. However, since the refractive power of the second lens group is too strong, aberration fluctuations in this second lens group upon zooming become larger, and the correcting of chromatic aberration also becomes difficult. As a result, a large amount of lower light-ray color flare occurs, especially at the long focal length extremity.
Compactness and low-cost are generally pursued at the design stage of a zoom lens system having a large number of lens groups. Therefore if an attempt is made to strengthen the refractive power of each lens group, or to reduce the number of lens elements, the amount of residual aberrations at each lens group increases, and the amount of aberration fluctuation upon zooming increases. Consequently, it is extremely difficult to attain an optimum optical quality, compactness and low production cost at the same time.